Heretofore, apparatus for detection of input signals in a noise environment utilized a transversal correlator, such as a noncoherent matched filter, to compare an incoming signal against a reference. The degree of correlation between the incoming signal and the reference produces an amplitude-varying analog output signal. This output signal peaks when the incoming signal substantially matches the reference. To detect the peak condition, a threshold detector responds to the output of the transversal correlator to generate a correlation pulse.
In such systems as heretofore utilized, knowledge of the exact timing synchronization was obtained. Due to channel disturbances, the signal entering the transversal correlator often contains noise components that result in an exact but false correlation occurring at some instant in nonsynchronized time. The probability of a false correlation is a function of both the signal-to-noise ratio at the transversal correlator input and the length of the correlator, that is, the length of the noncoherent matched filter. In such presently available systems the effective signal-to-noise ratio at the correlator output is increased over that of the input signal by an amount equal to the correlator processing gain. Thus, for a given input signal-to-noise ratio and correlator length, the probability of a false alarm (false correlation) is determined solely by the setting of the threshold detector level.
Since the probability of acquisition of a valid signal is given by a Marcum Q-function (for a noncoherent matched filter correlator, with white noise), and is equal to the probability of detection, a decrease in false alarm rate (i.e., a higher threshold setting) will result in an improved probability of acquisition. Thus, systems heretofore available for acquisition of signals in a Gaussian noise environment increased the probability of acquisition by either increasing signal power, increasing correlator length (and, therefore, processing gain), or both. A principal disadvantage of such prior art systems is the limitation in flexibility of conditions of varying signal-to-noise ratio and correlator lengths.
One of the major problems associated with automatic detection systems is the difficulty of detecting true input signals as distinguished from noise which is present in a Gaussian noise environment. The high noise environments may cause serious limitations in present day acquisition systems since a high noise condition may bring about total system breakdown due to the overloading of processing circuitry. Whenever this condition occurs, even very large true signals will be rejected due to the overload caused by noise signals.